Adaptive signal conditioning systems can be found in all areas of electronics and communication, and are generally used for adaptively conditioning the input signal to a signal conversion system such as an amplifier chain or any other suitable system in order to continuously provide a desired output signal of the overall system.
Predistortion is a signal conditioning technique, which is used in connection with for example power amplifier and transmitter systems, as described in references [1-9]. The main objective of predistortion is to compensate for distortion caused by the power amplifier or similar system by predistorting the input signal to the power amplifier with the “inverse” of the distortion characteristics of the power amplifier. Ideally, the cascaded response of the inverse predistortion function and the transfer function of the power amplifier results in an overall linear gain and phase transfer function. Typically, the inverse or complementary predistortion function is based on the approximation of the amplifier being modeled by a power series and characterized by its AM-AM (AM, Amplitude Modulation) and AM-PM (PM, Phase Modulation) characteristics. The inverse predistortion function may also include higher-order effects such as thermal properties of the power transistor and/or frequency-dependent properties due to the bias and matching circuitry. However, since the distortion caused by the power amplifier may change over time due to factors such as variations in ambient temperature and component aging, an adaptive predistortion scheme is employed to maintain the linearity. In general, an adaptive predistortion scheme requires a feedback from the output of the power amplifier and an associated adaptation control unit to keep track of changes in the transfer characteristics of the power amplifier and to adapt the predistortion function in response thereto.
FIG. 1 is a schematic block diagram illustrating a typical adaptive digital predistortion system applied in connection with a power amplifier. The input signal SIN is provided to a signal-conditioning block 110. The input signal is here assumed to be a digital signal, which has been subjected to conventional baseband processing. The signal conditioning block 110 implements a predistortion function and modifies the baseband data signal according to the predistortion function. The resulting predistorted signal is then converted into the analog domain in a digital-to-analog converter (DAC) 120 and up-converted to the radio frequency band in a frequency up-converter 130. Finally, the up-converted signal is amplified by the power amplifier (PA) 140 into an output signal SOUT, which is transmitted through an antenna 150.
The series connection of the digital-to-analog converter 120, the frequency up-converter 130 and the power amplifier 140 in the signal path after the signal conditioning block 110 is generally regarded as a signal conversion system. The predistortion function implemented in the signal conditioning block 110 generally represents the inverse of the distortion characteristics of the complete signal conversion system or appropriate parts thereof. In most cases, the power amplifier 150 stands for the dominant part of the distortion characteristics, and therefore the predistortion function is often provided as the inverse of the power amplifier distortion characteristics.
In order to enable adaptation of the predistortion function, a feedback path is arranged for providing an observed signal SOBS in response to the output signal SOUT of the power amplifier. The feedback path comprises a probe 160 for probing the power amplifier output, a frequency down-converter 170 and an analog-to-digital-converter (ADC) 180. The observed feedback signal SOBS is provided to a parameter adaptation unit 190, which adapts the parameters of the predistortion function based on the observed signal SOBS and a delayed version, using delay block 195, of the input signal SIN.
As long as the observed signal SOBS is an accurate representation of the output signal SOUT, the parameter adaptation will maintain an accurate and linear response of the forward transmission path. In practice, however, the transfer characteristics of the feedback path changes dynamically due to variations in temperature and frequency such that the observed signal SOBS at the output of the feedback path no longer is an accurate representation of the output signal SOUT. This may severely affect the overall performance of the adaptive predistortion technique. In fact, dynamic changes in the transfer characteristics of the feedback path is a key problem affecting the very core of any adaptive signal conditioning system.
For example, this problem manifests itself with respect to the need to maintain a specified signal level at the output of the power amplifier or similar signal conversion system. With reference once again to FIG. 1, it can be appreciated that the output signal SOUT is related to the input signal SIN and the transmission gain GTX in the following way:SOUT=SIN·GTX  (1)
Accordingly, it can be seen that the requirement of maintaining a specified output level can alternatively be expressed as maintaining a constant transmission gain GTX.
As pointed out above, the adaptive predistortion technique relies on the feedback path for providing an observed signal SOBS in response to the output signal SOUT, as well as the parameter adaptation in which the observed signal SOBS is compared to the delayed version of the input signal SIN with the goal of making SOBS equal to SIN. In practice, the feedback path has a gain GRX, and the relation between the observed signal SOBS and the output signal SOUT can be expressed as:SOBS=SOUT·GRX  (2)
By combining expressions (1) and (2), the following relation between the observed signal SOBS and the input signal SIN is obtained:SOBS=GRX·GTX·SIN  (3)
Since the goal of the parameter adaptation is to make SOBS equal to SIN, the parameter adaptation with respect to gain is working properly as long as the following holds true:GRX·GTX=1  (4)
The output signal SOUT is maintained at a specified level as long as both GRX and GTX do not change. However, in practice, both GRX and GTX change due to factors such as temperature variations and component aging. The gain factors GRX and GTX could possibly change in such a way that GRX·GTX=1. While this would not influence the predistortion parameter adaptation, it would result in an incorrect output signal since the altered transmission gain GTX=(GTXinitial+GTXchange) will be incorrect. Naturally, the gain factors GRX and GTX may change in such a way that GRX·GTX does not equal 1. This also results in an incorrect output signal SOUT.
In most applications, the system requirements on output power accuracy make it necessary to control the output signal level. Radio transmitters for example typically have requirements that the output signal level should be accurate within the range of +/−0.5 to +/−3.0 dB. The accuracy is especially important in CDMA systems where the output power of a base station or a terminal has to be controlled very accurately in order not to sacrifice system capacity. Due to radio transmitters being subject to high variations in ambient temperature and then including other effects such as aging, transmission frequency changes and power supply variations, there is a need to correct the gain variations of GRX and GTX.
With adaptive predistortion, the problem is generally simplified to keeping either GRX or GTX constant as the parameter adaptation is capable of keeping the other gain factor constant. However, if both GRX and GTX change, additional information and a corresponding adaptation process are required to correct for the second gain variation.
In this respect, a straightforward technique is to fully characterize either the transmission path or the feedback path over known variables such as temperature and transmission frequency at manufacture and then compensate for the gain variations by means of a gain-compensating device. It is known to use look-up tables that are addressed during operation to provide the required correction coefficients to the gain-compensating device. Another option is simply to select a suitable passive attenuator to compensate for the gain variations.
Pre-characterization however has the disadvantage that there is some uncertainty whether the gain compensation will remain accurate with the aging of the many components involved. Another disadvantage is the time required during production to complete the characterization and/or to calculate the compensation coefficients.
A more advanced technique, used in the commercially available radio base station RBS 1107/1127 from Ericsson, involves a transmission power tracking loop that maintains the gain of the transmission path based on actual measurements of the output power of the power amplifier and the input power to the adaptive predistortion system using dedicated power detectors. In this way, since the transmission path is gain calibrated, the parameter adaptation is capable of keeping the gain of the feedback path constant. However, the power detectors must still be calibrated over all known relevant variables such as power, temperature and frequency. Although the power detector components are relatively few compared to the complete transmission path, this represents tile same disadvantage as mentioned earlier, including uncertainty with aging and the time required in production.